The present invention relates to a rotor element and a method for producing the rotor element. The invention is primarily directed to a disk-shaped rotor comprising a plurality of circumferentially spaced rotor blades for a high-speed application.
The invention may be applied in a turbine rotor in an axial flow turbomachine, for example in a turbine of a turbopump for liquid rocket engines or in the fan of a high by-pass ratio turbofan aircraft engine. The rotor blades, like other high-speed aerofoils, are subject to an aeroelastic instability phenomenon known as flutter. Flutter is an aerodynamically self excited blade vibration problem which may arise when aerodynamic forces destabilizes the natural resonance of the turbine rotor during operation. Flutter is more likely to take place at higher aerodynamic loads and flow speeds.
In one type of bladed disk (blisk), two equal eigenmodes exist, for which identical but orthogonal mode shapes with equal natural frequency are separated by 90°. The natural frequencies interact during operation in a mutually amplifying manner, wherein flutter phenomena are encountered.
Flutter is undesirable because it causes extra stresses in the rotor. In turn, this may necessitate otherwise unnecessary rotational speed restrictions in order to ensure that the stresses in the rotor do not exceed desirable limits. Such restrictions prevent the full performance of the rotor being realized, with deleterious effects on available thrust and specific fuel consumption of the engine.
One known way of reducing the effect of flutter is to raise the natural frequencies or modifying modes so that aerodynamic stability is achieved. It is also known to add dampers, in the form of a ring damper or discrete dampers under the platform of the blades. An added damper has the effect of rubbing against the rotor body, wherein damping is achieved.
It is desirable to achieve a rotor element with improved characteristics with regard to inhibiting flutter during operation. Further, it is desirable to combat flutter and improving the aeroelastic stability of a rotor.
According to an aspect of the present invention, a rotor element comprises at least one irregularity with regard to rotational properties in its circumferential direction and that the irregularity is positioned so as to separate the mechanical natural frequencies of the rotor element.
Thus, the aeroelastic connection between the inherent natural frequencies of the rotor is disturbed. By virtue of the irregularity, the natural frequencies of the rotor will be separated so that at least one of them end up at a different natural frequency in relation to the inherent natural frequency. According to one example, a first of the natural frequencies is heavily effected and the other natural frequency is unaffected. Thus, the interaction between the natural frequencies of the rotor is disturbed.
In other words, the irregularity is adapted to effect the mechanical natural frequencies of the rotor, which is distinct from a conventional damper, which does not primarily seek to affect the mechanical natural frequencies of the rotor but instead only damps the magnitude of the natural frequencies. More particularly, by means of a damper, the internal relationship between the natural frequencies is not affected. Further, the interaction between the natural frequencies of the rotor is unaffected.
The term “rotor element” should be interpreted in a wide way, covering not only a rotor which is rotated in for example an aircraft engine in order to effect a gas flow, but also covering a separate element, such as a damper, which is attached to such a rotor. In both cases, the irregularity will effect the rotor in such a way that its natural frequencies are separated.
According to one aspect of the invention, the irregularity is formed in that different sectors in a circumferential direction of the rotor element has different mass and/or rigidity. In this way, the irregularity with regard to mass and/or rigidity causes the separation of the mechanical natural frequencies of the rotor element.
A “sector” is defined as a part of the rotor element defined between two planes which intersect in a central imaginary rotor element axis and which extend in an axial direction of the rotor element. Since the rotor element is circular, the sector has an arched outer periphery. In other words, in a plane perpendicular to the central imaginary rotor element axis, each sector is defined between two straight lines which intersect in the central imaginary rotor element axis and an outer arched line.
According to a further aspect of the invention, the irregularity is formed in that different sectors in a circumferential direction of the rotor element has different material thickness and/or composition and/or configuration. This may be achieved in several ways, for example by forming a rotor element body in a uniform material with different thickness in different sectors or that different sectors comprises different materials or a combination thereof.
According to a further aspect of the invention, the irregularity is positioned in a rotor element body portion of the rotor element. Thus, the rotor element body portion is modified so as to separate the inherent natural frequencies. This solution is distinct from separating the natural frequencies of different rotor blades. Thus, the invention is applicable to blisks, bladed disks and drums, and rotors without blades such as seals.
It is also desirable to achieve a method for producing a rotor with improved characteristics with regard to inhibiting flutter during operation.
According to an aspect of the present invention, the rotor element is formed with at least one irregularity with regard to rotational properties in its circumferential direction and positioning the irregularity so as to separate the mechanical natural frequencies of the rotor element.
According to one aspect of the invention, the method comprises the steps of forming the rotor element with an excess of material in at least one sector in a circumferential direction of the rotor element in a first step and removing, part of said excess of material by machining in a second step. This is preferably achieved by forming the rotor element by means of turning a work piece in the first step and by milling or grinding off the excess material in the second step.
The rotor configuration may be optimized for applications to a given turbine rotor by use of techniques and mathematical analysis which are within the skill of the art.
Further advantageous embodiments and advantages of the invention will be apparent from the following description, and drawings.